1. Field of the Invention
The invention relates to refractory dielectric bodies, in particular glass optical fiber preforms.
2. Discussion of the Related Art
Optical fiber is produced from a glass preform. As discussed in F. DiMarcello et al. "Fiber Drawing and Strength Properties," Optical Fiber Communications, Vol. 1, Academic Press, Inc., 1995, at 179-248, the disclosure of which is hereby incorporated by reference, the preform is generally arranged vertically in a draw tower such that a portion of the preform is lowered into a furnace region. The portion of the preform placed into the furnace region begins to soften, and the lower end of the preform forms what is known as the neck-down region, where glass flows from the original cross-sectional area of the preform to the desired cross-sectional area of the fiber. From the lower tip of this neck-down region, the optical fiber is drawn.
The optical fiber typically contains a high-purity silica glass core optionally doped with a refractive index-raising element such as germanium, an inner cladding of high-purity silica glass optionally doped with a refractive index-lowering element such as fluorine, and an outer cladding of undoped silica glass. In some manufacturing processes, the preforms for making such fiber are fabricated by forming an overcladding tube for the outer cladding, and separately forming a rod containing the core material and inner cladding material. Overcladding tubes are capable of being formed by a sol-gel process, as discussed, for example, in U.S. Pat. No. 5,240,488, the disclosure of which is hereby incorporated by reference. It is also possible to form overcladding tubes by drawing the tubes from a silica billet, and such tubes are available commercially. The core/inner cladding rods are fabricated by any of a variety of vapor deposition methods known to those skilled in the art, including vapor axial deposition (VAD), outside vapor deposition (OVD), and modified chemical vapor deposition (MCVD). MCVD is discussed in U.S. Pat. Nos. 4,217,027; 4,262,035; and 4,909,816, the disclosures of which are hereby incorporated by reference. MCVD involves passing a high-purity gas, e.g., a mixture of gases containing silicon and germanium, through the interior of a silica tube (known as the substrate tube) while heating the outside of the tube with a traversing oxy-hydrogen torch. In the heated area of the tube, a gas phase reaction occurs that deposits particles on the tube wall. This deposit, which forms ahead of the torch, is sintered as the torch passes over it. The process is repeated in successive passes until the requisite quantity of silica and/or germanium-doped silica is deposited. Once deposition is complete, the body is heated to collapse the substrate tube and obtain a consolidated rod in which the substrate tube constitutes the outer portion of the inner cladding material. To obtain a finished preform, the overcladding tube is typically placed over the core rod, and the components are heated and collapsed into a solid, consolidated preform, as discussed in U.S. Pat. No. 4,775,401, the disclosure of which is hereby incorporated by reference. Prior to collapsing the tube onto the rod, it is sometimes desirable to treat the outer surface of the rod and/or the inner surface of the overcladding tube, e.g., by cleaning with solvents, fire polishing, and or etching. Some of these treatments, however, are harsher than desired in that they require removal of some surface material and/or involve mechanical stress or chemical attack.
While current methods for treating preform components and forming preforms are adequate, improvements and/or alternatives are desired.